Leiomyosarcomas: three cases with desmin positive tumour cells, lacking ultrastructural features of smooth muscle cells.

A combined study of light and electron microscopy and of immunolabelling of three pleomorphic spindle cell sarcomas is presented. The light and electron microscopic features of these sarcomas were most compatible with those described for malignant fibrous histiocytoma (MFH, pleomorphic-storiform subtype). Electronmicroscopically undifferentiated and fibroblast-like cells, fibrohistiocytes and multinucleated histiocytes were observed. Characteristics belonging to smooth muscle cells were absent. By immunostaining, vimentin and desmin could be observed in tumour cells of all three cases, at least on frozen sections. Other markers such as alpha 1-antichymotrypsin, S-100 proteins, laminin, collagen IV and markers specific for skeletal muscle cells (myoglobin, actin and myosin specific for skeletal muscle) could not be demonstrated. These findings indicate that three MFH's are, in fact, poorly differentiated variants of smooth muscle tumours. It is concluded that immunophenotyping is very useful for this type of neoplasm.